STUDY OF THERMAL PROPERTIES, FORCED CONVECTION & PRESSURE DROP IN Al2O3 & TiO2 NANOFLUIDS

Abstract

The present research investigates the heat transfer and pressure drop characteristics of Al2O3 and TiO2/water nanofluids. All the aspects of nanofluids are covered in the study starting from measuring the thermal conductivity using DOE (design of experiments) approach, stability analysis and finally forced convection heat transfer and pressure drop analysis. In this study, spiral tape inserts are also used to enhance the heat transfer. Thermal conductivity is very important and desirable property which favors heat transfer and viscosity rise is undesirable, which increases pressure drop. Both the properties are opposite in nature and behave differently with the variation in particle size, particle concentration and temperature. So, to analyze the simultaneous effect of different variables and to optimize the properties for maximum thermal conductivity and minimum viscosity, the DOE approach and utility concept is used. A full factorial design is used for this analysis. It is observed that, the thermal conductivity of the nanofluids increases with increasing particle concentration and temperature while by increasing particle diameter the thermal conductivity decreases. A significant interaction effect is observed between the particle concentration and temperature. Thermal conductivity is high when both concentration and temperature are at their higher levels. The maximum thermal conductivity enhancement is observed to be 11.51 % when the particle concentration is 1 vol %, particle diameter 20 nm and temperature 40 ℃. Minimum viscosity rise is observed to be 0.24 % at the particle concentration of 0.1 vol %, particle diameter of 20 nm and temperature 40. It is important to collectively optimize the thermal conductivity and viscosity of nanofluids. Multi-response optimization is achieved by using utility concept. The composite optimal condition is achieved at a particle concentration of 1 vol %, 20 nm diameter and 40 ℃ temperature. The enhancement in thermal conductivity at optimal condition is 11.51 % and the rise in viscosity is 6.3 %. In the present study, zeta potential is measured as an indicator of suspension stability. A high absolute value of zeta potential is the key indicator of the stability. In the present study the effect of particle concentration, sonication time, pH and the effect of surfactant is analyzed on the zeta potential of Al2O3/water nanofluids. It is observed that the zeta potential is maximum at 0.1 vol. % and 60 minutes sonication time. At high concentration (0.5 vol. % and 0.8 vol. %), value of zeta II potential is maximum at 30 minute and minimum at 180 minutes. Comparing particle concentrations (0.1, 0.5 and 0.8 vol %), the zeta potential is maximum for 0.1 vol % for all sonication times. The particle concentration varied from 0.01 to 0.8 vol %, sonication time is varied from 30 to 180 min, pH is varied from 2.5 ± 0.2 to 11.5 ± 0.2. The zeta potential of the nanofluids is higher in the acidic region than that of the basic region, at all the sonication times. The isoelectric point (IEP - point at which the zeta potential becomes zero) for all the particle concentrations is found at the pH value of 8.6 for sonication times of 120 and 180 minutes, whereas it varies between 8 to 9.4 for the sonication time of 60 minutes. One of the main objectives of the present investigation is to determine experimentally the distribution of local and average heat transfer coefficients and Nusselt numbers for the distilled water, Al2O3/water and TiO2/water nanofluids in a Cu horizontal pipe heated by a nichrome heater under uniform heat flux boundary condition. So that the heat transfer performance of nanofluids can be analyzed and compared with that of distilled water. For this an experimental setup was designed and fabricated. It consists of a flow loop including a pump, a flow measuring device, a test section with heating element, a cooling unit and a reservoir. Following observations were investigated. No magic enhancement with Al2O3 nanofluids was observed when the nanofluid was used in plain tube, compared with distilled water. Rather deterioration was observed with nanofluids. The heat transfer decreases with increase in particle concentration. The maximum deterioration in Nusselt number was observed to be 14.31 % at 0.1 vol %. Two types of spiral tapes are used to enhance the heat transfer; simple and modified spiral tapes for twist ratio of 3.04, 4.35 and 5.65. Simple spiral tape inserts are efficient to enhance the heat transfer but with the increase in penalty of pressure drop. With water as a working fluid a 12.7 % increase in heat transfer is observed using 5.65 twist ratio simple spiral tape insert. Further enhancement in heat transfer is possible with modified spiral tape but at extra cost of pumping power. Thermal performance factor analysis shows that modified twisted tapes with twist ratios of 3.04 and 4.35 effectively enhanced the heat transfer without extra penalty of puming power.